Interesting read for people wanting to better understand this term that is used often. This was posted by DT a few years back on wxrisk.com:

THE 50-50 LOW....

What it is... why it is important... what it does

In my forecasts... discussions and essays that talk of the Risk threat or Likelihood of Significant winter weather for the eastern US in general and the Middle Atlantic/ Northeast in particular. I often make references to something called the 50-50 LOW. This often draws a response from weather hobbyists and professional meteorologists alike.... What does the term "50/50 Low" refer to? WHY is it important? and what is its significance?

The term "50-50 Low" refers to the development or appearance of a strong or intense Low-pressure area both at the surface and in upper levels of the atmosphere -- usually at 500 MB -- over or near the positions of 50 degrees North Latitude and 50 degrees West Longitude. Hence the name "50-50 Low". If you look at a map you'll see that position of 50N Latitude and 50 West Longitude is also close to the province of the Newfoundland Canada.

Sometimes a forecaster may refer to the NF or Newfoundland Low. The term "NF Low" is identical in every way to the term 50-50 Low.

As most snow lovers can tell you whether its the Middle Atlantic region or the Northeast U.S. most snowstorms along East Coast and specifically in the I-95 corridor from Richmond to Boston feature either a changeover to Rain or at least mixed precipitation. Inland of course it's the changeover often does not occur and this is reflected in the climatology which shows inland locations from Southwest Virginia all the way into central New Hampshire receive two to three times as much snow as coastal areas. Occasionally however... the conditions and all the pieces will fall into place that will support a Significant East Coast Snowstorm (what I call "S.E.C.S".... Major East Coast Snowstorm or M.E.C.S.).

Many forecasters and weather hobbyists are familiar with terms such as the Negative NAO (-NAO) and the +PNA pattern. However the set up for SECS / MECS is far more complicated than just getting those two important teleconnections to set up in the right location and phase. In the general sense the appearance of a 50-50 Low is favorable for the pattern to develop a significant East Coast snowstorm. That being said of course it is also true that having a 50-50 Low does NOT guarantee a SECS anymore that having a -NAO does. There are many many cases where having a -NAO or 50-50 Low does NOT result in anything close to a significant East Coast snowstorm.

However having both of these features together IS a significant indication to experienced forecasters that the pattern is setting up to become favorable for a SECS / MECS . It is the relationship between the -NAO ... the Polar Vortex (PV)... and the 50/50 Low that is critical in determining what the overall setup is going to be and which area (Northeast US Middle Atlantic US or both) is more likely to be affected in the event that a SECS does indeed develop.

The appearance of a 50-50 Low affects the overall pattern across eastern North America in several ways.

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1. THE NAO...

There are several of the various manifestations of the phenomena known as the Negative NAO:

A- IRELAND / -NAO: The height anomaly seen on the Upper level maps may be placed as far east as IRELAND and still qualify as a -NAO.

B- GREENLAND BLOCK /-NAO: The height anomaly is over central Greenland--- which would qualify the -NAO. Not all -NAO are Greenland blocks!

Here is an example of a Greenland block from Accuweather...

Cold air will start to infiltrate the region later tonight and through the weekend. Accuweather depicted a "Greenland Block" in the image above. Basically, It's when a blocking area high pressure is situated over Greenland and forces the jet south. Almost like a rock in a stream. Canada has a ton of cold air just sitting over it, once the block over Greenland takes shape, it will free the cold air over the eastern two-thirds of the United States.

C) DAVIS Straits -LABRADOR / -NAO: The height anomaly appears as far west as the Davis Straits into Labrador and northeast Quebec.

So one of things a 50-50 Low does is it often enhances the intensity or amplitude of the -NAO in general and the Greenland Block -NAO in particular.

(You have to keep in mind these features are in a symbiotic relationship... the glory and the power of meteorology is that it reminds all of us that weather is a very connected dynamic type of science. We have the tendency to teach science in the Western world as compartmentalized areas of study. This often results in some rather unusual jealousy and rivalry -- mathematicians who will not talk to physicists... astronomers who will talk to those who are engaged in SETI research. )

At any rate sometimes the appearance and development of a 50-50 Low will shorten the wavelength in the PJ (polar jet) across Northwestern Atlantic to such a degree that it causes a Neutral NAO to "go Negative"...

... or sometimes a strong negative NAO will force a ordinary storm in the Canadian Maritimes to stall and deepen and become the new 50-50 Low.

2. COLD AIR SOURCE FOR THE EASTERN US

The ONLY way that the major cities of the I-95 corridor can maintain an all snow event is for the cold air Source to be of sufficient magnitude and location to allow the ageostrophic circulation to set up and keep the cold air flowing all the way to the coast during the snowstorm.

This the second way the 50-50 low is critical for SECS / MECS is in the synoptic setup. The 50/50 Low keeps the cold air source--- the Arctic or semi Arctic High over the Great Lakes or southeastern Canada --trapped and locked into place.

3.PREVENTS SYSTEMS IN PLAINS/MIDWEST FROM PASSING WEST OF THE MOUNTAINS

The third way the 50-50 low is critical for the development of East Coast snowstorms... is that it forces potential systems (i.e. short waves) that drop out of central Canada (Miller B systems) into the Upper Plains and track further to the South and East since they have to travel underneath and around the 50-50 Low. OR in the case of a Deep South Low pressure areas (Miller A systems) the 50-50 Low prevents Southern from tracking west of the the Appalachian mountains.